Copyright = ownership of game code and assets due to having created them. Trademark is an assertion of ownership of the name and right to sell specified goods under that marque with certain protections. Both can be transferred by contract or other agreement.
Specifically, Interplay can sell something that is (c) somebody else if there is an agreement for them to do so with the developer, the exact terms under which it is done varies so it may be no money to the developer except during actual development, a proportion of sales revenue or bonus payments for sales milestones. There's usually no way to know.
When a company owns both (c) and TM it is usually because of one of three reasons. Firstly, they are self publishing ('indies'). They may also have a formal publisher (= 'distributor') such as Paradox for Pillars of Eternity, but that publisher owns no formal rights to the title except the right to sell.
Secondly, the creator may be an independent studio doing work for hire, this was the usual case up until fairly recently and would have something like Fallout: New Vegas as an example; Obsidian did the work but Bethesda owns the IP. A slightly different example would be Alpha Protocol where Obsidian started making the game and when Sega came on board the contract stated that the rights would lie with them rather than Obsidian.
The third possibility is simply that the publisher owns the developer, so something like Dragon Age: Origins is both TM and (c) EA because they own Bioware outright. If EA bought Obsidian then Pillars of Eternity 2 would be in the same boat.
If a company holds both TM and (c) it usually means that they own the Intellectual Property outright so can sell, alter the product, do sequels etc. If they don't it usually means that they either cannot do anything or require permission from others to do anything.
